What happened
On the night of 30 June 2023, a Bathawk unmanned aircraft system (UAS), registration ZT-UOZ, was conducting a night-time surveillance mission for Transnet Freight Rail in Westonaria, Gauteng. The operation was being performed under beyond visual line of sight (BVLOS) regulations. Following a successful initial flight, the pilot prepared for a second mission by replacing the battery, which showed 98% power at launch.
During the flight, the aircraft reached an altitude of 400 feet above ground level. As the mission progressed, wind conditions shifted, with speeds increasing to 21 knots. The pilot observed the aircraft repeatedly yawing and attempting to correct itself, prompting a decision to initiate the return-to-lag function. However, as the UAS approached the base station, it descended to 150 feet and began oscillating heavily with a significant roll. Despite the pilot engaging loiter mode to stabilize the aircraft, the UAS entered a series of spins before impacting the terrain approximately 360 metres north of the ground station. The impact resulted in substantial damage to the aircraft, though there were no injuries to anyone on the ground.
The investigation
An investigation into the crash examined flight logs and technical data from the operator. The analysis of the aircraft's logs revealed that while the system recorded consistent airspeed throughout much of the flight, a critical discrepancy occurred during the return phase. The investigation established that the aircraft's airspeed sensor provided inaccurate readings during the return-to-home stage.
Data indicated that as the aircraft attempted to maintain altitude, the automated flight system increased the angle of attack. This, combined with changing wind conditions and battery-related factors, led to a reduction in actual ground speed and an eventual aerodynamic stall.